Don't look now, but another big chunk of the "evidence" for man-made global warming suddenly disappeared. Poof! Researchers just reported that the world's most recent case of "abrupt climate change"—which occurred a mere 12,000 years ago—was probably due to a comet strike, not to "climate sensitivity."

The Younger Dryas occurred as an Ice Age was ending. As the climate began to warm, a huge and sudden rush of fresh meltwater broke out from the Great Lakes and swept out to sea. The water surge was monumental enough that the meltwater lowered the salinity of the ocean, shut down the Atlantic conveyor currents, which disperse the planet's heat, and threw the northern hemisphere back into another thousand years of Ice Age. It raised temperatures near Greenland by a startling 15 degrees C, even as it doubled annual rainfall.

Modern climatologists have savored the Younger Dryas event as massive evidence of what comes when we push the planet's climate too close to a "tipping point." Further human-driven warming, they say, will make such abrupt climate changes more likely, with searing droughts, torrential rainfall, and extreme heat.

The National Academy of Sciences issued a 2002 report titled Abrupt Climate Change: Inevitable Surprises, which said abrupt climate changes have been especially common when the climate system was being forced to change most rapidly. According to that theory, greenhouse warming today could be drastically increasing risks from climate change.

At least, that's what the experts said until the latest meeting of the American Geophysical Union in Acapulco on May 23rd when James Kennett of the University of California/Santa Barbara presented evidence of a dramatically different cause for the Younger Dryas event: a comet that struck somewhere near the Great Lakes.

"Highest concentrations of extraterrestrial impact materials occur in the Great Lakes area and spread out from there," Kennett says. "It would have had major effects on humans. Immediate effects would have been in the North and East, producing shockwaves, heat, flooding, wildfires, and a destruction and fragmentation of the human population."

Paleontologists had assumed a huge lake of meltwater accumulated near the Great Lakes due to the Ice Age ending, but had never located its possible site. Nor have they explained a thin layer of charred sediment found throughout North America that dates from 12,000 years ago. The sediment layer contains carbon spheres whose creation would have required temperatures of at least 4000 C. Electron microscopes reveal that the carbon beads contain tiny diamonds whose creation would have required enormous temperatures and pressures.

The U.S. sediment layer does not contain much iridium, which is the telltale signal of an asteroid strike. That argues for a comet, made up primarily of "dirty ice," rather than an asteroid like the one which hit Mexico's Yucatan Peninsula 65 million years ago and wiped out the dinosaurs.

Kennett says the ice sheet could have absorbed the impact of the comet's "dirty ice," even as the comet's heat produced the flood of meltwater. Kennett says the comet may have destroyed 15 mammal species and might have left only a few surviving humans from North America's early Clovis culture. America's bison survived, but much smaller in size and with a remarkable similarity in their DNA—indicating that they descended from a small group of comet survivors.

The comet theory comes as a crushing blow to the climate alarmists. It follows the publication of Unstoppable Global Warming—Every 1,500 Years, which assembles the historic and scientific evidence of a long, natural climate cycle that swings temperatures about 2-4 degrees C over its lifetime—accounting for the Medieval Warming, the Roman Warming and the Holocene Warming 5,000 years ago. Then came Henrik Svensmark's demonstration at the Danish Space Research Institute, of how cosmic rays link changes in the sun's irradiance to the formation of the low, wet clouds that cover more than 20 percent of the earth. The clouds are nature's thermostats, deflecting more or less heat back out to space depending on the sun's strength.

Now the alarmists have lost the "abrupt climate change" of the Younger Dryas. More and more, recent science is pointing to our modern warming as being part of a 1500-year cycle that stretches back at least a million years.

If the Younger Dryas was caused by a comet, perhaps we should rethink being frightened by the neighbor's SUV.

Dennis T. Avery was a senior policy analyst for the U.S. State Department, where he won the National Intelligence Medal of Achievement. He is the co-author, with atmospheric physicist Fred Singer, of the book, Unstoppable Global Warming—Every 1500 Years, available from Rowman & Littlefield. Readers may write him at the Center for Global Food Issues, Post Office Box 202, Churchville, VA 24421